Method and system for predicting air discharge temperature in a control system which controls an automotive HVAC system

ABSTRACT

Method and system for predicting air discharge temperature in a control system which controls an automotive HVAC (heating, ventilation and air conditioning) system using a model. The model is based on signals generated by various climate control sensors and actuation outputs to predict air discharge temperature without the need for an additional sensor. The model allows the avoidance of uncomfortable air discharge temperatures to thereby increase passenger comfort.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to U.S. patent applications entitled "MethodAnd Control System For Controlling An Automotive HVAC System", "MethodAnd Control System For Controlling An Automotive HVAC System To PreventThe Discharge Of Air Within A Predetermined Temperature Range" and"Method And System For Modifying A Linear Control Algorithm WhichControls An Automotive HVAC System" all of which have the same inventiveentity, are assigned to the same assignee and have the same filing date.

TECHNICAL FIELD

This invention relates to methods and systems for predicting airdischarge temperature in a control system which controls an automotiveHVAC system and, in particular, to methods and systems for predictingair discharge temperature in a control system which controls anautomotive HVAC system using a model.

BACKGROUND ART

A fundamental goal of automotive heating, ventilating, and airconditioning (HVAC) systems is to make vehicle occupants comfortable. Toachieve this goal, it is important that the design of the control systemthat establishes cabin conditions takes into account the relationshipbetween comfort and the variables that affect comfort. Human comfort isa complex reaction, involving physical, biological, and psychologicalresponses to the given conditions. Because of this complexity, theengineer must consider many variables and their possible interaction inthe design strategy of such a control system or controller.

In an attempt to measure and control the many variables that affectcomfort, modern automotive HVAC systems have many sensors and controlactuators. A typical system might have a temperature sensor inside thecabin, one measuring ambient temperature outside and others measuringvarious temperatures of the system internal workings. The occupant mayhave some input to the system via a set point or other adjustment.Additional sensors measuring sun heating load, humidity, etc. might beavailable to the system. The set of actuators might include a variablespeed blower, some means for varying air temperature, ducting and doorsto control the direction of air flow and the ratio of fresh torecirculated air.

It falls to the controller to sort out the range of possible conditions,determine what is needed to achieve comfort, and coordinate the controlof the set of actuators available. This multiple input, multiple outputcontrol problem does not fall into any convenient category oftraditional control theory. The performance criterion, comfort, is notsome well defined formula but a sometimes inconsistent goal, empiricallydetermined. In particular, comfort control is not the same astemperature control. The response of the system as well as therelationship between system variables and desired performance, comfort,is rarely linear. Also, it is important to note that despite all theactuators and variables available for control, there may existconditions under which comfort may not be achievable.

Due to practical considerations of size, energy consumption, cost andthe wide conceivable range of conditions that automobiles are exposedto, the system plant may simply not be able to supply what is needed.All these considerations lead to a control problem that is far from whatis usually encountered in traditional control theory.

In the face of these difficulties, most control system designs have usedwhat is familiar--linear control--and supplemented it by patched-inspecific responses to handle special circumstances where necessary. Inother words, typical automobile automatic climate control systems uselinear proportional control to maintain a comfortable interiorenvironment. However, there are two significant limitations of linearproportional control when viewed from the standpoint of an occupant'ssubjective comfort: first, there are certain control situations in anyHVAC system that are inherently nonlinear, and second, it is notpossible to realize occupant comfort merely by maintaining proximity toa desired temperature as described in greater detail hereinbelow.

The design of a typical HVAC climate control system starts with the needto provide acceptable occupant comfort levels under the most extremehigh and low ambient conditions that a vehicle might encounter. Forthese conditions, the control system is requesting the HVAC unit tooperate at peak output in one direction or the other. Designconsiderations center around plant capacity and the efficiency of heattransfer in order to handle these extremes. The control system iseffectively saturated until one or more of the input signals indicatethat some level of comfort control is achievable.

It is at this point that the system begins to moderate its control ofblower speed, the location of discharge air (mode of operation), and therelative blend of cooled and heated air. The simplest approach tocontrol in this region is to have the control follow a straight linebetween the two extremes. Such a linear control algorithm adjusts theoutputs in an appropriate manner and its parameters are easy todetermine based on the points of onset of the two extreme regions. Witha well defined HVAC system and enough developmental evaluation time, onecan tune the coefficients to provide acceptable levels of comfort for avariety of operating conditions. The linear approach is quite wellunderstood and easy to implement. For a small microprocessor-basedcontroller, its essence is captured in a few lines of code.

The linear approach has obvious limitations when dealing with nonlinearsituations. All HVAC systems behave nonlinearly in various regions oftheir operation. The transfer of heat as a function of blower speed isnonlinear. The onset of any plant output limitation affects desiredresponse in a nonlinear fashion. Factors affecting plant limitations maybe tracked via additional sensors--for example, engine coolanttemperature (ECT) correlates with heater core temperature--but again,the relationship is nonlinear. The usual approach to handling specialnonlinear situations is to use logic-based modification of the usuallinear strategy when these situations are detected. Thus, in coldweather, when ECT is below a certain threshold indicating that theheater core cannot warm the cabin, the blower would be shut off.

This particular solution to the problem of nonlinearities createsproblems of its own. In the case of the binary ECT threshold switch,interaction with the linear strategy leads to difficulties. When thethreshold ECT is passed, the switch turns on the blower. Since the caris cold, the blower immediately goes to its highest setting and createstwo problems. The first is the noise level produced by the bloweroperating full out. The second problem is that all the residual cold airin the system is blown directly onto the customer's feet causingdiscomfort.

In addition to the current difficulties, new vehicle lines createadditional problems that are not easy to overcome. The reduction ininterior and under hood package space in current vehicle designs hascaused the transfer function for discharge temperature to be even morenonlinear, especially when operating at the extremes of ambienttemperature.

The response of crisp (as opposed to fuzzy) logic in a control strategydoes not fit well when human comfort is the goal. Abrupt changes inenvironment are not perceived favorably by most people. It is true thatthe effect of sudden changes occasioned by crisp logic transitions maybe masked via input or output filtering. Also, some of the resultingconditions may not be experienced by the occupant as a level ofdiscomfort. For example, heater warmup, linear or nonlinear, has noeffect on comfort on a hot day with the system at maximum cooling.

There are some aspects of the strategy for automatic vehicle climatecontrol that are most naturally conditioned on the value of the airdischarge temperature. Most control algorithms must take into accountthat certain discharge temperatures are not experienced as being verypleasant. Without an actual discharge temperature measurement, thesealgorithms take certain actions when the blend door is within aparticular range. Unfortunately, blend door position does not completelypredict discharge temperature.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a model-based methodand system for predicting air discharge temperature in a control systemwhich controls an automotive HVAC system. Preferably, the model-basedmethod and system are based on relevant sensor measurements as well asblend door position and blower speed to more accurately predictdischarge temperature to allow better control of passenger comfort.

In carrying out the above object and other objects of the presentinvention, a method is provided for predicting air discharge temperaturein a control system which, in turn, controls a heating, ventilation andair conditioning (HVAC) system of a vehicle which discharges a parcel ofair to a passenger cabin of the vehicle. The system includes a variablespeed blower, means for varying air temperature, ducting, actuatorsincluding a blend door having control positions for controlling thedirection of air flow and the ratio of fresh air to recirculated air andsensors for sensing temperature within the cabin and ambienttemperature. The method includes the step of determining at least oneinteraction time of a parcel of air with the means for varying airtemperature based on the speed of the blower. The method also includesthe step of calculating the temperature of the parcel of air dischargedto the passenger cabin based on: (1) the interaction time, (2) thetemperature of the means for varying air temperature, (3) a controlposition of the blend door, and (4) the temperature of the parcel of airprior to interaction with the means for varying air temperature.

Further in carrying out the above object and other objects of thepresent invention, a system is provided for carrying out each of theabove method steps.

The above objects and other objects, features, and advantages of thepresent invention are readily apparent from the following detaileddescription of the best mode for carrying out the invention when takenin connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating an air handling system to becontrolled;

FIG. 2 is a schematic block diagram of the control system to control thesystem of FIG. 1;

FIGS. 3a through 3e are graphical illustrations of a blower speed ruleset including antecedent and corresponding consequent membershipfunctions;

FIG. 4 is a graphical response surface resulting from the rule set ofFIGS. 3a through 3e;

FIGS. 5a through 5e are graphical illustrations of a mode shift rule setincluding antecedent and corresponding membership functions;

FIG. 6 is a graphical response surface resulting from the rule set ofFIGS. 5a through 5e;

FIGS. 7a through 7h are graphical illustrations of an offset rule setincluding antecedent and corresponding membership functions;

FIGS. 8a through 8c are graphical illustrations of a target adjust ruleset including antecedent and corresponding membership functions;

FIG. 9 is a graph of a variable target adjust as a function of ambienttemperature;

FIGS. 10a through 10c are graphical illustrations of a gain variable,G₃, rule set including antecedent and corresponding consequentmembership functions;

FIGS. 11a through 11c are graphical illustrations of a gain variable,G₄, rule set including antecedent and corresponding consequentmembership functions;

FIG. 12 is a graph of discharge temperature versus blend door positionwith excluded temperatures being illustrated by a hysteresis loop;

FIG. 13 are graphs of blower motor voltage and blend door positionsversus in-car temperature;

FIG. 14 are graphs of cabin temperature versus ambient temperature forthe prior art and the present invention at 75 degree set point;

FIG. 15 is a graph of blower speed versus engine coolant temperatureduring heater warm-up;

FIG. 16 is a schematic diagram of a model of the air handling system ofFIG. 1; and

FIG. 17 is a schematic diagram of an example of a single interactionbetween a parcel of air having an input temperature and a part of theair handling system.

BEST MODE FOR CARRYING OUT THE INVENTION

In general, control of temperature within an automobile is accomplishedusing various actuators to adjust the temperature and flow of airsupplied to the cabin of the vehicle. FIG. 1 shows schematically an airhandling system of an HVAC (heating, ventilation and air conditioning)system, generally indicated at 20. The system 20 includes thearrangement of panel-defrost, floor-panel, temperature blend and outsiderecirc air actuators or doors 22, 24, 26 and 28, respectively. The doors22, 24 and 28 are driven by vacuum motors (not shown) between theirvarious vacuum, partial vacuum and no vacuum positions in a conventionalfashion as indicated in FIG. 1. The door 26 is driven by an electricservo motor also in a conventional fashion.

The system 20 also includes a variable speed blower motor or fan 30including a blower wheel 32.

The system further includes heating and cooling elements such as aheater core 34 and an evaporator core 36 in a typical vehicle airconditioning plant. Each of the above components is in communicationwith ducting 38 in order to control temperature, the direction of airflow and the ratio of fresh air to recirculated air.

For automatic control of the temperature and flow of air in the cabin,conditions within and outside the cabin are monitored by sensors and anelectronic controller generates signals to control the plant actuatorsaccording to the conditions as indicated by the sensors. As illustratedin FIG. 2, a typical complement of sensors of the HVAC system providesignals which are representative of in-car temperature, ambient(outside) air temperature, engine coolant temperature (ECT), dischargeair temperature and sunload. However, as described below, the method andsystem of the present invention eliminates the need for a discharge airtemperature sensor. In addition, there is a set signal or settemperature value indicating the desired temperature that is setmanually by the driver. In turn, an incar-set temperature (in-car minusset temperature) signal and a set-75 (set temperature minus 75 degreesFahrenheit) signal are generated or calculated.

The signals are provided to an electronic controller 40 as inputs afterbeing conditioned by a conditioning circuit 42. The controller 40 scalesthe input signals and provides scaled output signals for use by ahardware controller 44 which, in turn, controls the doors 22 through 28to regulate the temperature and flow of air and ultimately to maintainthe comfort of driver and passengers in the vehicle.

Referring to FIG. 16, there is illustrated a discharge temperature modelbased on a physical model of the climate control plant of FIG. 1. It isassumed that the process used to obtain a parcel of air at a particulartemperature can be broken up into a series of interactions of thatparcel and various elements of the system. FIG. 17 gives an example ofone such interaction between a parcel of air at temperature T_(in) and aplant element at temperature T_(element) The temperature of the inputair "decays" toward the temperature of the element, depending on howlong the air is in contact with the element. The resulting temperatureof the air after the interaction is given by:

    T.sub.out =T.sub.in +(T.sub.element -T.sub.in) * (1-e.sup.Δt/t1)

where T_(element) is the temperature of the component, Δt is aninteraction time, and t₁ is a time constant for the interaction. Thisformula assures that the resulting temperature is between T_(element)and T_(in). The interaction time is a function of blower speed: onewould normally expect interaction time to be inversely proportional toblower speed, but in the case of interaction with the heater core 34 thedata indicate a direct proportionality.

The system of FIG. 16 starts with air at either T_(ambient) orT_(incar), depending on the position of the recirculation door 28. Thisair interacts first with the A/C core or evaporator 36 (assuming notemperature change through the blower wheel 32) and then is divided intotwo paths by the blend door 26. One fraction of the air interacts withthe heater core 34 and the remainder passes by to rejoin and mix withthe heated air. One assumes the fraction of air routed to the heatercore 34 is directly proportional to the blend door position (however,any more appropriate function could be substituted therefor). The finalinteraction is with the walls of the discharge ducting beyond the blenddoor 26.

Assuming the walls are at T_(incar), interaction time, as well asT_(wall), depends on door positions. The various parameters of thismodel are chosen either by common sense or by hand fitting to data. Forexample, one uses 44 deg as T_(evaporator), ECT as T_(heater), and asmentioned, T_(incar) as T_(wall). The time constants for theinteractions are all hand fit. For example, the data indicate t₁ islarge, i.e., T_(B) ≦T_(evaporator). As mentioned before, interactiontime is proportional to blower speed in the case of the heater core 34.

The fitting of the parameters could also be done via a more rigorousprocedure such as least squares.

The calculated value for discharge temperature from this model isfiltered with a 15 second time constant to give the final value used bythe preferred control strategy as described below. This approachobviates the need for an additional sensor but still gives dischargetemperature within a few degrees of actual values.

A control strategy or algorithm is a name for the mathematical processthat takes sensor inputs and produces control outputs. There isdescribed in detail below a control strategy based on fuzzy logic asillustrated by rule sets 46 in FIG. 2 for control of temperatureregulation and passenger comfort.

In the block diagram of the fuzzy logic system of FIG. 2, sensor inputsare conditioned, scaled, and passed to a fuzzy inference engine of thecontroller 40. Rule sets for the various control functions--blowerspeed, offset, target set point, mode shift, recirculation/fresh modeshift, term calibrations, etc. provide the inference engine with thedetails of the strategy to be performed. The fuzzy outputs are scaledand passed to the routines to control the motors, actuators, and doorsthat accomplish the flow and conditioning of the air supplied to thepassenger compartment. The rule set basis for control organizes thestrategy in a manner that allows easy calibration and adjustment of thecontrol operation.

Control algorithms should take into account that a range of dischargetemperatures is not experienced as being very pleasant. The usualapproach is to "hide" the air produce by the plant when its temperatureis within a certain range. The air is "hidden" by directing it partiallythrough the defrost outlet and partially to the floor rather thanthrough the panel. The present application describes a means whereby anuncomfortable range of temperatures is not produced at all, yetregulation and passenger comfort are maintained.

The exclude air algorithm operates by holding discharge air temperaturejust below the lower limit of the uncomfortable range when this regionis approached from below and holding discharge temperature just abovethe uncomfortable range when approached from above. FIG. 12 is a graphof the dependence of discharge air temperature as a function of blenddoor position (with other variables held constant) showing the region ofexcluded temperatures. The approach to this region is detected by any ofa number of methods: actual measurement of discharge temperature,modeling of discharge temperature from the sensor and actuatorinformation known, or blend door attaining particular upper or lowerpositions for certain conditions of ambient temperature and set pointtemperatures. As in FIG. 2, blend door position is fed back to thecontroller 44 by way of a return arrow.

Implementation of this strategy is by calculation of a variable calledfuzzy₋₋ mode, conditioned on ambient temperature and regulation error asindicated in FIG. 6 wherein other variables such as sunload anddischarge temperature are assumed to be constant. The value of thisvariable determines when mode transitions occur (as indicated by thearrows in FIG. 12). If the current mode is floor and this variableexceeds a certain threshold, the mode switches to panel (i.e. vacuummotor moves floor-panel door 24 to its NV position with thepanel-defrost door in its V position). If the current mode is panel andthis variable drops below a certain threshold, the mode switches tofloor (i.e. vacuum motor moves floor-panel door 24 to its V positionwith the panel-defrost door 22 to its V position).

FIG. 6 shows the value of fuzzy₋₋ mode as a function of regulation errorand ambient temperature (these are shown scaled between -1 and 1 ratherthan their actual ranges of -20 to +20 for regulation error and 10 and120 for ambient temperature). The L-shaped flat region in the middle ofthe surface evaluating to zero. The size of this region determines thehysteresis in switching between modes.

Equation 1 below shows a prior art linear control formula. The controlvalue calculated is scaled and used for both blower speed and blend dooractuation. The coefficients, K₁, K₂, K₃, and K₄ are constant gains thatmust be calibrated to compensate for the effect of their respectiveterms on the resulting control.

    Control.sub.-- Value=Offset--K.sub.1 * SUN+K.sub.2 * (Set.sub.-- Point--75)+K.sub.3 * (75-Ambient)+K.sub.4 * (Set.sub.-- Point-Incar)(1)

In fuzzy-linear control of the present application, the calculationtakes the same form as in Equation 1 above, only some of thecoefficients and variables are now fuzzy output variables. Consider, forexample,

    FL.sub.-- Value=Offset--G.sub.1 * SUN+G.sub.2 * (Set.sub.-- Point--75)+G.sub.3 * (75-Ambient)+G.sub.4 * (Target-Incar),(2)

where all the underlined symbols represent fuzzy output variablescalculated as functions of the sensor inputs. By "fuzzy outputvariable," one means variables calculated from the sensor inputsaccording to the usual fuzzy logic Max-min algorithm as described in theabove-noted paper of Mamdani using a set of rules. This form obviouslysubsumes linear control in its possible control behavior, but it can beappropriately nonlinear and also includes direct fuzzy control as isdescribed hereinbelow.

Choosing any of the various gains to be fuzzy variables allowscalibration blending. Suppose one obtained calibration values for G₄ of0.45 during winter tests, 0.50 during the spring, and 0.60 for thesummer. The fuzzy evaluation process illustrated in FIGS. 11a through11c could be used to blend these calibration values in a reasonablefashion as a function of ambient temperature. No one value ofcalibration need be agreed upon and chosen, instead ambient temperatureis used as an indicator of which calibrations come into play indetermining the value of G₄.

The fuzzy variable, "Target," in Equation 2 can be used to compensatefor variation of control most logically associated with set pointschanges, either from physical, mechanical, or even psychological causes.For example, suppose it is determined that what most customers mean by a72 degree set point is "comfortable", a set point of just that value formost circumstances--but for different seasons that translates intodifferent temperatures. In winter "comfortable" might mean an actualtemperature of 75 degrees even though the customer setting is at 72degrees. In summer, it might be an actual value of 68 degrees for asetting of 72. FIGS. 11a through 11c illustrate how this customer"offset-in-meaning" might be compensated for via the Target fuzzyvariable. One chooses Target=Set₋₋ Point+Target Adjust, where TargetAdjust behaves according to the graph.

The fuzzy variable, Offset, in Equation 2 is used to compensate for allother nonlinear effects that do not fall easily into the abovecategories. FIGS. 7a-7h show the membership functions which define theoffsets. If, for example, for cold-weather starting, one wanted to blockair flow over the heater core 34 to allow the engine to warm up morequickly, Offset would be chosen to be a fuzzy variable of enginetemperature. The value of Offset would be blended between its usualcalibration value and a value that would fully block the heater core 34as a function of engine temperature in the same way gains were blendedas a function of ambient temperature.

The fuzzy output variable G₃ in Equation 2 is used to compensate for ahigher rate of loss of heat when the outside temperature is very cold.One simply adds a rule stating that if AMB is LOW, then G₃ is high(relative to its normal value).

All these features provide the advantages of nonlinear control that, forexample, direct fuzzy logic control could provide, but with a simpler,more natural organization that will allow easier calibration andadjustment with attendant shortening of development time.

Referring again to FIGS. 1 and 2 in combination with FIGS. 3a-3e andFIG. 4, the desired blower speed of an automobile heating/airconditioning system 20 can be considered a function of temperature error(in-car temperature--set point temperature) and engine coolanttemperature (ECT). If the error is small, low blower speed is desired.If the error is positive and high (it's hot inside), high speed isneeded to cool the cabin down. If the error is negative (it's coldinside) and the engine is cold, a little speed is needed for defrost butif the engine is warm, high speed is needed to heat up the cabin. Thedescriptions "small error", "high speed", etc., are defined by themembership functions in the set of rules shown in FIGS. 3a-3e.

In FIGS. 3a-3e, the degree to which a rule holds is computed from theantecedent membership functions on the left which are dependent on theirrespective input values. The consequent membership functions on theright define the degree of control action (blower speed) to be takenwhen the antecedent condition pertains.

The heuristic rules for blower speed are as follows:

1. If Incar is close to Set Temperature, then the blower tends toward alow speed;

2. If Incar-Set is high, then the blower tends toward a high speed;

3. If the Ambient Temperature is very hot or very cold and Incar isclose to Set Temperature, then the blower tends toward a medium speed;

4. If Incar-Set is negative and Engine Coolant Temperature is high, thenthe blower tends toward a high speed; and

5. If the Ambient Temperature is low and Engine Coolant Temperature islow, then the blower tends toward a low speed.

In this way, blower speed varies with ECT (100°-180° F.); blower speedslowly increases to clear residual air unnoticeably; and noise from theblower is reduced due to smooth increase in speed.

Evaluating the five rules gives the response shown in FIG. 4. In otherwords, FIG. 4 illustrates the response surface resulting from the rulesets in FIGS. 3a through 3e. Blower speed as a function of enginecoolant temperature (ECT) and in-car minus set point temperaturedifference. The temperature difference and ECT are in units of degreesFahrenheit, and blower speed is scaled to blower fan volts. This rathernonlinear response makes sense for each of the conditions described inthe rules stated, but smoothly interpolates between the regions.

Fuzzy logic is naturally nonlinear. Linear control is subsumed by fuzzycontrol, and it is possible to make fuzzy control linear, if wanted. Inmany control problems, nonlinearity is associated with difficulties. Thenonlinearities arising from fuzzy control, however, follow naturallyfrom the logic of the strategy desired. If the strategy is appropriateto the problem, there should be no particular difficulty with theresulting nonlinear response. Any of a number of other methodologies,for example, lookup tables, could produce the same desired response thatone sees in FIG. 4. On the other hand, the descriptive organization thatleads to the response is particularly simple and understandable in thecase of fuzzy logic. It is the organization of the control strategy inthe form of a set of rules that makes the fuzzy logic strategy easy tounderstand and maintain.

Another feature of the fuzzy control algorithm is that the response inany particular corner (say, in this example, the amount of blower speedfor defrost) may be adjusted separately, without affecting the responseelsewhere. If some area of the control space requires unique attention,a rule may be added to provide sufficient control without affecting thecontrol action in other areas of the control space.

Referring again to FIG. 2, the fuzzy logic climate controller 40preferably uses a Motorola 68HC11 microprocessor for its calculations.This microprocessor has 512 bytes of RAM and 12 kilobytes of ROM builtin. It uses an 8 megahertz clock providing a 500 nanosecond instructioncycle time. An eight channel analog-to-digital converter is integratedinto the microprocessor. Four of the eight channels are used to measureinputs that are used by the control system, namely: ambient (outside)temperature, engine coolant temperature, interior temperature, andsunload. A further input to the system is the set point temperaturewhich may be adjusted by the vehicle occupants using buttons on thefront face of the control unit. The system outputs are: intake air mode,discharge air mode (fresh air or recirculate), blend door position, andblower speed. The latter two outputs are continuous-valued, the formerdiscrete. The fuzzy logic control calculation takes scaled input valuesand produces a single relative output value. Since there are four systemoutputs, there are four rule sets. For the continuous-valued outputs,the fuzzy logic output value is scaled and used directly; for thediscrete outputs, the output values are compared to thresholds togenerate particular modes or states of the system.

The controller 40 is preferably programmed primarily in C andcross-assembled into microprocessor instructions. Each fuzzy rule set isincorporated into the fuzzy engine as a set of tables that have beenpreviously converted into a form that allows for efficient calculationduring run time. The fuzzy logic control procedure is called as part ofthe main loop, which is executed every 30 milliseconds. The fuzzy logicengine occupies approximately 600 bytes of ROM and uses 12 bytes of RAMduring its execution. Execution time for a fuzzy calculation istypically 20 milliseconds.

Referring again to FIG. 2, the main inputs to the fuzzy engine areincar-set temperature difference and absolute engine coolanttemperature. Set or target temperature is obtained from a linearcombination of incar-set, 75-ambient, and set-75 temperaturedifferences, along with input from the sunload sensor. Targettemperature is defined as the incar temperature which the system istrying to reach to produce the zero error.

During hot engine operation, the blower voltage versus targettemperature is roughly shaped as a "V" curve with the minimum in blowervoltage occurring at target temperature. As incar temperature deviatesfrom target temperature, blower voltage increases on either side oftarget. Through shaping of the fuzzy logic input membership curves,optimum blower voltage is obtained for any incar-target temperatureerror.

During warm-up from a cold start, under heating conditions, the fuzzyrules obtaining their input from engine coolant temperature come intoaction. If ECT is less than CELO temp (110° F.), the blower voltage isheld at a minimum and the defrost mode is in effect. AT CELOtemperature, a mode shift to mixed mode occurs. Mixed mode stays ineffect for 10 seconds and is then followed by floor mode. Above CELOtemp, and up to 180° F., there is a gradual increase in the bloweroutput, finally reaching a maximum value that would be normally obtainedunder hot engine conditions. This blower ramping is controlled throughshaping of the fuzzy logic rules accepting ECT and incar-targettemperature error.

The blend door strategy is implemented as a linear combination ofincar-set, 75-ambient, set-75 and sunload values. The blend door actionis enhanced through a feature that excludes certain discharge airtemperatures. Because it is undesirable to discharge warm air out thepanel ducts and cold air from the floor ducts, the blend door positionis locked at certain positions to prevent improper discharge airtemperatures when approaching mode transitions. These locked positionstrack with mode shift hysteresis and produce a coordinated controlbetween discharge air temperatures and mode shifting.

In addition, in order to better coordinate discharge air temperatureswith mode shifting, mixed mode can only be entered in automatic modeduring the floor to panel mode shift and not the panel to floor modeshift. During mode shifts, the secondary door positions are slightlydelayed in time so that the blend door has the opportunity to reach itsnew target position before actual mode transitions occur.

Referring now to FIGS. 5a through 5e, the desired mode for directing airin the HVAC system 20 can be considered a function of ambienttemperature and temperature error (linear temperature--set pointtemperature). Descriptions are defined by the membership functions inthe set of heuristic rules shown in FIGS. 5a through 5e and as follows:

1. If Incar-Set is high, then mode tends toward Panel;

2. If Incar-Set is low, then mode tends toward Floor;

3. If Incar is close to Set temp, then mode tends toward the middle ofFloor and Panel;

4. If Incar-Set is low and Ambient Temperature is low, the mode tendstoward Floor; and

5. If Incar-Set is high and Ambient Temperature is high, then mode tendstoward Panel.

Evaluating the five rules gives the response shown in FIG. 6.

The method and system of the present invention provide numerousadvantages. For example, occupant comfort is able to be maintained morereliably over a greater range of conditions. In particular, the comfortrating as ambient temperature varies is judged noticeably superior tothe prior art linear strategy. FIG. 14 shows the response of the systemto a steady increase in ambient temperature and how it variesconsiderably from the existing linear strategy. It matches at theextreme and mid-range temperatures but has been adjusted through testingto improve comfort and respond differently elsewhere.

The system is also able to address particular customer concerns incertain regions of operation. Blower speed oscillations are removed,thus diminishing a problem with erratic blower noise. Another area ofcustomer concern, that of blower speed onset in cold weather has beendiscussed above. The method and system responds to the slow rise of theengine coolant temperature by bringing up blower speed slowly, allowingthe system to unnoticeably rid the ducts of the residual cold air andgradually bring the blower to its ideal speed as shown in FIGS. 3a-3e.The shape of the blower speed onset curve can be tailored by adjustingthe membership functions of FIGS. 3a-3e.

The use of the present invention in climate control systems strategyresults in improved occupant comfort. The ability to tailor gradual,nonlinear response has allowed the design of the strategy to addresscertain situations that have not been handed gracefully in the past. Inparticular, concerns such as blower speed onset during warmup in coldweather and ambient temperature compensation have been ameliorated byappropriate use of the additional flexibility that the present inventionprovides.

New vehicle programs and the requirements of simultaneous engineeringare reducing the time available to develop new automatic climate controlstrategies. Consequently, the control strategy should be developed atthe same time that the HVAC system is being designed. The organizationand flexibility of the present invention allow one to develop a set ofbase rules even before the characteristics of a HVAC system have beenfinalized.

While the best mode for carrying out the invention has been described indetail, those familiar with the art to which this invention relates willrecognize various alternative designs and embodiments for practicing theinvention as defined by the following claims.

We claim:
 1. A method for predicting air discharge temperature in acontrol system which, in turn, controls a heating, ventilation and airconditioning (HVAC) system of a vehicle which discharges a parcel of airto a passenger cabin of the vehicle, the system including a variablespeed blower, means for carrying air temperature, ducting, actuatorsincluding a blend door having control positions for controlling thedirection of air flow and the ratio of fresh air to recirculated air andsensors for sensing temperature within the cabin and ambienttemperature, the method comprising the steps of:sensing temperature ofthe means for varying air temperature; determining speed of the blower;determining a control position of the blend door; determiningtemperature of the parcel of air prior to interaction with the means forvarying air temperature; determining at least one interaction time ofthe parcel of air with the means for varying air temperature based onthe speed of the blower; and calculating the temperature of the parcelof air discharged to the passenger cabin based on: (1) the at least oneinteraction time, (2) the temperature of the means for varying airtemperature, (3) the control position of the blend door, and (4) thetemperature of the parcel of air prior to interaction with the means forvarying air temperature.
 2. The method as claimed in claim 1 wherein thetemperature of the air discharged to the passenger cabin is anexponential function of the at least one interaction time.
 3. The methodas claimed in claim 1 wherein the means for varying air temperatureincludes a heating element and wherein a first interaction time of theparcel of air with the heating element is determined wherein the step ofcalculating is based on the first interaction time, the temperature ofthe heating element and the temperature of the parcel of air prior tointeraction with the heating element.
 4. The method as claimed in claim3 wherein the means for varying air temperature includes a coolingelement and wherein a second interaction time of the parcel of air withthe cooling element is determined, the control position of the blenddoor controlling the amount of the parcel of air interacting with theheating and cooling elements and wherein the step of calculating is alsobased on the second interaction time, the temperature of the coolingelement and the temperature of the parcel of air prior to interactionwith the cooling element.
 5. The method as claimed in claim 4 whereinthe means for varying air temperature further includes walls of theducting, a third interaction time of the parcel of air with the walls ofthe ducting being determined by the step of determining, wherein thestep of calculating is also based on the third interaction time, thetemperature of the walls of the ducting and the temperature of theparcel of air prior to interaction with the walls of the ducting.
 6. Themethod as claimed in claim 5 wherein the temperature of the walls of theducting is substantially equal to the temperature within the cabin. 7.The method as claimed in claim 5 wherein the actuators include anoutside/recirc. air door for establishing a ratio of fresh air torecirculated air to control the temperature of the parcel of air priorto interaction with the means for varying air temperature.
 8. A systemfor predicting air discharge temperature in a control system which, inturn, controls a heating, ventilation and air conditioning (HVAC) systemof a vehicle which discharges a parcel of air to a passenger cabin ofthe vehicle, the HVAC system including a variable speed blower, meansfor varying air temperature, ducting, actuators including a blend doorhaving various control positions for controlling the direction of airflow and the ratio of fresh air to recirculated air and sensors forsensing temperature within the cabin and ambient temperature, the systemcomprising:a sensor for sensing temperature of the means for varying airtemperature; means for determining speed of the blower; means fordetermining a position for the blend door; means for determiningtemperature of the parcel of air prior to interaction with the means forvarying air temperature; means for determining at least one interactiontime for the parcel of air with the means for varying air temperaturebased on the speed of the blower; and means for calculating thetemperature of the parcel of air discharged to the passenger cabin basedon: (1) the at least one interaction time, (2) the temperature of themeans for varying air temperature, (3) a control position of the blenddoor, and (4) the temperature of the parcel of air prior to interactionwith the means for varying air temperature.
 9. The system as claimed inclaim 8 wherein the temperature of the air discharged to the passengercabin is an exponential function of the at least one interaction time.10. The system as claimed in claim 8 wherein the means for varying airtemperature includes a heating element and wherein a first interactiontime of the parcel of air with the heating element is determined whereinthe calculated temperature is based on the first interaction time, thetemperature of the heating element and the temperature of the parcel ofair prior to interaction with the heating element.
 11. The system asclaimed in claim 10 wherein the means for varying air temperatureincludes a cooling element and wherein a second interaction time of theparcel of air with the cooling element is determined, the controlposition of the blend door controlling the amount of the parcel of airinteracting with the heating and cooling elements and wherein thecalculated temperature is also based on the second interaction time, thetemperature of the cooling element and the temperature of the parcel ofair prior to interaction with the cooling element.
 12. The system asclaimed in claim 10 wherein the means for varying air temperaturefurther includes walls of the ducting, a third interaction time of theparcel of air with the walls of the ducting being determined by themeans for determining, wherein the calculated temperature is also basedon the third interaction time, the temperature of the walls of theducting and the temperature of the parcel of air prior to interactionwith the walls of the ducting.
 13. The system as claimed in claim 12wherein the temperature of the walls of the ducting is substantiallyequal to the temperature within the cabin.
 14. The system as claimed inclaim 12 wherein the actuators include an outside/recirc. air door forestablishing a ratio of fresh air to recirculated air to control thetemperature of the parcel of air prior to interaction with the means forvarying air temperature.